If you're the account owner, connect with a client or command line using the account owner's username and password; cd public_html (or dbl click on the public_html folder).

If you're another user, you have to have ftp access rights. Login with username@domain.tld and password. You'll be directed to the folder created for that user unless that user was given htdoc root access "/" (in otherwords, you may not have access to the root of public_html).

I've also seen where FTP needs to be re-synched in WHM before it will accept connections.